Highly purified steam is required for various medical purposes, such as production of pharmaceuticals, demanding sterilization applications and production of water for injection. A method and device for production of such steam are disclosed in U.S. Pat. No. 3,875,017. A falling film evaporator as disclosed therein comprises a vertical bundle of evaporation tubes enclosed into a heating jacket, which in turn is enclosed by an outer shell so that an annular space is formed between the heating jacket and the outer shell. Water fed into the evaporating tubes at their upper end flows down the inner surface of the tubes, thereby evaporating and forming steam, which emerges at the lower ends of the tube bundle. The flow of steam makes a 180° turn and flows upwards in the space between the heating jacket and the outer shell. Fins forming a spiral path are attached to the outer surface of the heating jacket, leaving a narrow gap between their edges and the inner surface of the shell. The steam flowing upwards is forced into a spiraling path, whereby water droplets in the evaporation product are driven towards the outer shell by centrifugal force. The droplets adhere to the outer shell wall and form a film of water flowing downwards and finally forming a pool of liquid at the bottom of the device. From there, a stream of water proportional to the amount of purified steam produced is withdrawn as a reject stream. As the water phase in the evaporation product tends to be rich in impurities, these are enriched into the reject stream. From above the spiral path, purified steam is led to the consumption points or to a condenser for producing highly purified water.
In International patent application WO 02/24299, a device of a similar type is disclosed, having apertures in the outer wall of the spiral path. Outside said apertures, a cooled surface is provided. This arrangement creates a pressure gradient across the apertures due to condensation of steam on the cooled surface, causing impurity-containing droplets to migrate through the apertures and collect on the cooled surface. The least pure component of the rising stream in the spiral path thus travels to the periphery and leaves through the apertures, condenses on the cooled surface and forms a water phase rich in impurities, which flows down and mixes with a pool of liquid at the bottom of the device below the zone where the steam makes its 180° turn referred to above. From this pool, a reject stream is withdrawn.